Newton's Principia: Astronomical prerequisites? I am planning to read the Principia but I currently have no knowledge about Astronomy. What astronomy prerequisites do I need to understand the Principia? I would prefer older books (or "treatises"), anything from 1600's+, as I don't really like the modern textbook style. I heard that studying Copernican astronomy is a good idea, but the only books I know on this are Copernicus's book (English translation available) and Gassendi's Astronomy (No english translation; I can't read latin)
 A: A few weeks ago I was at a rare books store in Las Vegas and was able to page through a second edition copy of Principia from 1723. Just flipping (carefully, it was a $15,000 book) to random pages landed me on:


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*Newton showing that the same force that pulls apples to the ground also keeps the moon in orbit.

*Newton describing how light diffracts when encountering a narrow slit.

*And, for good measure, Newton inventing calculus.


Unfortunately, the book was in Latin (the original language of science), so I couldn't get much detail out of it (not to mention the shop keepers getting more nervous with every page turn). In any case, from what I could discern from the words I recognized and the diagrams, not much astronomy knowledge is needed. What concerned Newton was the motion of the planets, sun, and moon across the sky (that is, angular measurements similar to longitude and latitude) and how their real motion could be derived and used to prove his ideas for how gravity worked. If you can follow geometric reasoning--like how parallax works--then you'll be fine.
Back in Newton's time, almost nothing was known about what the lights in the sky were. Some lights moved with respect to other lights, some didn't. The near surface of the moon was pretty well mapped out because it's so close, but the composition and nature of other heavenly bodies was unknown. The geometry of their motion was the research area of the day to explain things like the retrograde motion of Mars. The elliptical shapes of orbits had been puzzled out by Kepler, but the details of their motion required Newton's ideas on gravity.
Other than brushing up on trigonometry and other geometry, dive in.
A: I think you need english classes more than anything. I tried to read the to english translated version (by Andrew Motte) just before I started my university studies...well let's just say that I didn't make it through the motivation for the translation, let alone reading the actual text. 
In any case, from what I remember there were 
a lot of geometry, which really surprised me because back then I thought Newton would introduce and use Calculus extensively (to my best knowledge, he later did this in Optiks). 
Also there were some tables of the planets. 
It was really nice to just have that book, because I wanted to have the feeling that I was reading what Newton actually wrote, but then I realized that, hey, I'm reading a translation, Newton wrote it in Latin, so that was a fail in many ways I guess haha.
I might give it another try when I retire in 100 years.  
